The invention relates to an apparatus for contacting a gas with a liquid, in particular for aerating waste water. More in particular it relates to an apparatus comprising a surface aerator adapted to rotate about a vertical axis and a driving means coupled with said aerator.
Such apparatuses are being used successfully for a long time in installations for treating waste water. In further developments of these apparatuses it was proposed according to the British patent specification No. 1,547,115 of applicant to dispose a rigid cap over the surface aerator, the cap having a downwardly directed circumferential side wall which terminates, during operating, below the liquid surface, and having a gas inlet opening with an adjustable cross-section. By the use of said cap, the nuisance due to the splashing of the surface aerator, and to the noise generated by the same was effectively reduced and in addition, if a stench was produced, this was arrested to a very considerable extent by the cap.
Although the cap, of course, impedes the gas supply to the surface aerator, it had been surprisingly found that the cap causes a very significant improvement of the oxygen introduction efficiency. This improvement appeared to be due to a different operation of the surface aerator in the closed space at a reduced gas supply. By the presence of the cap the outflow of the gas bubbles produced by the aerator is slowed down. Furthermore, throwing out liquid by the surface aerator against the inner side of the cap causes a high rate of turbulence, which improves the transfer of oxygen of the gas bubbles to the liquid. This optimal condition can be recognized in the appearance of foam along the inner side of the side wall of the cap as a consequency of the accumulation of gas bubbles.